Text: António Piedade | Photography: DR
Since the dawn of Christendom, the date of Easter, the day on which Christ’s resurrection is celebrated, has been fundamental to the structuring of the entire Christian liturgical calendar.
But the unequivocal determination of Easter Day so that it could be celebrated on the same day of the calendar by all of Christendom, regardless of its geographical location, was a problem that was only normalized in the first ecumenical council that took place in Nica in 325 AD
At this council, convened by the Roman emperor Constantine, it was determined that Easter day should be celebrated on the first Sunday after the first full moon that occurred at (or shortly after) the spring equinox in the northern hemisphere – about 21 March. This has been the rule since then to determine Easter day and, therefore, the Moon will always be in the full phase.
the determination of the day of the Full Moon, for the determination of the Easter Sunday, makes use of what is defined in the Ecclesiastical Tables
But the determination of the equinox, through the calendar then followed, did not guarantee a “coincidence” between the forecast and the reality, due to the imperfection contained therein. The Julian calendar (so named in honor of Julius Caesar) in effect at the time of the council of Nicaea accumulated an imprecision of about 11 minutes and 14 seconds in excess each year.
Around 1582, the inaccuracy of the Julian calendar resulted in the spring equinox occurring on the 11th instead of March 21st as one would expect. This gap introduced errors in the Christian religious calendar and, in practice, Easter Day was celebrated on different days in different parts of the hemisphere. Something needed to be done to re-adjust the official calendar.
Pope Gregory XIII (1502 – 1585) created a commission led by the Jesuit mathematician and astronomer Christoph Clavius (1537-1612) to solve the problem.
In his bull Inter Gravissimas, Pope Gregory XIII enshrines mathematical work and institutionalizes the calendar that we still follow in the West today and that bears his name (Gregorian calendar). It results from a very satisfactory set of rules for regular hits in the so-called leap years, which ensures an acceptable compromise in the prediction of the relative translation movements of the Earth around the Sun and the Moon around the Earth.
It should be added, however, that the determination of the Full Moon day, for the determination of Easter Sunday, does not make use of the astronomical tables, but of the one defined in the Ecclesiastical Tables, which, although they do not accurately include the complex movement of the orbit of the Moon, are sufficient to allow a regular and uniform determination of the same moment throughout Western Christendom, regardless of its latitude and longitude.
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